There's a lot of definitions for proof:
1a:
the cogency of evidence that compels acceptance by the mind of a truth or a fact
b:the process or an instance of establishing the validity of a statement especially by derivation from other statements in accordance with principles of reasoning
2obsolete :
experience
3:
something that induces certainty or establishes validity
4:archaic :the quality or state of having been tested or tried; especially :unyielding hardness
5:evidence operating to determine the finding or judgment of a tribunal
6:a copy (as of typeset text) made for examination or correction:a test impression of an engraving, etching, or lithograph:a coin that is struck from a highly polished die on a polished planchet, is not intended for circulation, and sometimes differs in metallic content from coins of identical design struck for circulation:a test photographic print made from a negative
7:a test applied to articles or substances to determine whether they are of standard or satisfactory quality
8a:the minimum alcoholic strength of proof spirit
b :strength with reference to the standard for proof spirit; specifically :alcoholic strength indicated by a number that is twice the percent by volume of alcohol present whiskey of 90 proof is 45 percent alcohol
I'd go with 1. Not quite 3.